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Effect of visible light on photo-stimulated-phosphor imaging plates
International Congress Series 1256 (2003) 1199 – 1205
Effect of visible light on photo-stimulated-phosphor imaging plates Roberto Molteni * Gendex Dental X-ray Division, Dentsply Gendex, 901 W. Oakton Street, Des Plaines, IL 60018-1884, USA Received 18 March 2003; received in revised form 18 March 2003; accepted 20 March 2003
Abstract When irradiated with X-ray radiation, photo-stimulated-phosphor (PSP) imaging plates (IP) charge up with a latent radiographic image. Such image is discharged, hence (partially) erased, on exposure to visible light. In this work, the effects of exposure to light are quantitatively investigated and reported, in order to provide proper guidelines for (a) handling radiographically exposed imaging plates prior to scanning, so as to prevent excessive fading of the latent image and loss of information due to ambient light, and (b) removing the remaining latent image prior to utilising the imaging plate for a further X-ray examination. Data are reported for three types of light and two types of photo-stimulable phosphor. D 2003 Published by Elsevier Science B.V. Keywords: Storage phosphor; Imaging plates; Photo-stimulated phosphors; Photo-stimulable-phosphor radiography
1. Introduction and purpose When irradiated with X-ray radiation, photo-stimulated-phosphor (PSP) imaging plates (IP), a.k.a. storage phosphors, charge up with a latent radiographic image. This virtual, latent image is physically constituted by the local trapping of electrons into metastable electronic levels, whose spatial density is directly proportional to the absorbed dose over a very broad range [1]. In the dark, the trapped electrons will ultimately leak from the trapping energetic states and the latent image will naturally fade away over a period of several weeks. However, the image can be quickly discharged, hence (partially) erased, on exposure to visible light. * Tel.: +1-847-640-4922; fax: +1-847-640-4970. E-mail address: [email protected] (R. Molteni). 0531-5131/03 D 2003 Published by Elsevier Science B.V. doi:10.1016/S0531-5131(03)00345-5
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Light from any part of the visible spectrum is variedly effective at erasing the PSP; however, according to data published by the PSP manufacturer [2], the most efficient erasing occurs with light at approximately 600 nm wavelength (red). Imaging plates have been used for many years for medical applications as a part of dedicated scanning systems that handle the imaging plate inside of a light-tight cassette, and that automatically erase the remaining signal or latent image at the conclusion of a scanning. In dental radiology, however, the simpler (and less expensive) systems that have become commercially available during the past few years usually rely on manual handling both for loading the imaging plate into the scanning device, and for erasing any remaining latent image after the scanning is performed. Therefore, the effects of exposure to light need to be quantitatively investigated in order to determine:
The procedures to properly handle X-ray-exposed IP prior to scanning to prevent excessive fading of the latent image, and loss of information, due to ambient light. The procedures to properly remove the remaining latent image prior to utilising the imaging plate for a further X-ray examination.
2. Materials and methods Dental-specific PSP imaging plates (IP) have been used for the investigation:
Fuji BAS in intraoral size #2 Fuji ST-V, which is used for panoramic (15 30 cm, 5 12 in.) and cephalometric (18 24 cm, 8 10 in.) sizes.
Both PSP materials are originally manufactured by Fuji Films, Tokyo, Japan, and finished and branded by Dentsply Gendex, Des Plaines, IL, USA, for use with the DenOptix dental X-ray scanning system. Imaging plates made of the same, or similar, material are used in various other commercial systems, for instance, ScanX marketed by AirTechnique and DigoraR produced by Soredex. In addition to size #2 intraoral imaging plates (which are made out of BAS material), a panoramic-size imaging plate, which is made out of ST-V material, was cut into #2-size plates, for convenience and consistence of the tests. In this way, the panoramic- and cephalometric-specific PSP-IP material could be handled in the same manner as the intraoral-specific PSP-IP material, i.e. be subject to the same X-ray irradiation procedure with the same generator and test object, and scanned and erased in the same way as the (BAS) intraoral IP. The imaging plates were irradiated with a controlled amount of X-ray radiation dose, in a dedicated fixture, which consisted of a mount to ensure repeatable exposure geometry at a source-detector distance (SDD) of 30 cm. The fixture included a very simple test object that consisted of a thin (0.15 mm) straight lamina of lead vertically slanting at a small angle at one side of the exposed field on the imaging plate. This test object is not relevant for the purpose of the investigation reported herewith, except for the convenience of making images immediately recognisable.
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The 765DC dental intraoral X-ray generator (Dentsply Gendex, Des Plaines, IL, USA) was used. The 765DC features 65 kV and 7 mA at constant potential (DC), and was equipped with a 30 cm rectangular collimator (note that the presence and nature of the collimator are irrelevant for the purpose of this test). Due to their design, DC-operated Xray generators can ensure consistent exposure and a very linear time– dose relationship even at short exposure times. The Test-O-Meter type 511 radiation dose meter from Unifors Instruments (Billdal, Sweden) was used for measurements of dose. The imaging plates were scanned with DenOptix (Dentsply Gendex, Des Plaines, IL, USA), SCSI version, and the resulting images were analysed through the VixWin2000 software (Dentsply Gendex, Milan, Italy). This system, if properly configured, can acquire the signal in a strictly linear fashion, and the software itself provides the tools to quantitatively analyse it. Such configuration implies the following menu settings: ‘‘Options\Preferences\8 – 16 bit’’: 16 bit. ‘‘Options\Automatic Image Treatment\DenOptix I/O’’: ‘‘Raw Image’’ on, ‘‘Equalize’’ off. The irradiated images were (partially) erased by exposing to three different types of light sources (with different light spectra), i.e. ‘‘cool white’’ fluorescent light tubes (for general illumination), ‘‘aquarium’’ fluorescent light tubes (purple light), and incandescent filament bulbs. A Gossen Multi-Pro photometer was used for light measurements. The measures were also double-checked with an INS DX-100 Lux-meter. Initially, a test was conducted to determine the reproducibility in the response of the PSP-IP when exposed to consistent X-ray exposure in the measurement setup, and using different pieces of imaging plates. A test was then conducted to verify the linearity of the exposure dose from the 765DC Xray generator as a function of exposure time. Then, the linearity of the response of the PSP-IP as a function of exposure time was measured, for both the BAS type and the ST-V type. The ‘‘response’’ is defined as the ‘‘signal’’ readout of a rectangular region of the exposed and scanned IP, which has been subject to (approximately) uniform X-ray irradiation. This is obtained by complementing to 65535 (64k = 216 1) the mean value of the signal read over the ROI, i.e. the 16-bit digital image ‘‘density’’, due to the fact that the digital image presented is a negative or complementary image (as customary in radiography). Since this test was run in comparable conditions for both PSP-IP types, the result also provides their relative sensitivity. The a.m. tests are just preparatory to the tests that constitute the actual scope of this investigation, i.e. to quantify the effect of visible light to fading and/or erasing the latent signal stored in the PSP-IP. BAS type imaging plates were exposed to a predefined X-ray dose, i.e. 0.87 mGy (obtained with an exposure time of 200 ms in the test fixture). As reported in Section 3, this resulted in a latent signal of about 80% of the full-scale range. The procedure was repeated with ST-V type imaging plates, however, with an exposure time of 40 ms resulting in 0.17 mGy and an image signal comparable to the one above.
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The preexposed IP were then subject to visible light of different spectrum, i.e. from:
‘‘Cool white’’ fluorescent tubes, Radionic Industries type F14T8CW; ‘‘Aquarium’’ fluorescent tubes, Radionic Industries type F14T8AQ. These are purple in colour, with a higher content of short wavelengths; Incandescent filament bulb lamps. Light exposure/erasing was performed at three different light intensities:
10,000 lx, this being a bright light intensity typical of light erasing fixtures, set at very close distance from a bank of fluorescent tubes. Exposure to light was performed in a time range up to 30 s; 600 lx, this being representative of a well-lit office environment. Exposure to light was performed in a time range up to 60 s; 20 lx, this being illustrative of dim, or subdued, light conditions as prescribed by the manufacturer instructions [3] for handling the PSP-IPs after X-ray exposure and prior to the scanning and image readout, e.g. typically achievable in a windowless unlit room where light enters through a door open onto a regularly lit office environment. Exposure to light was performed in a time range up to 300 s. 3. Results At a exposure dose of 0.7 mGy (corresponding to 160 ms) and 12 samples, the average signal was 56.0% of full scale, with a standard deviation of 1.3% of full scale, and 2.3% of the signal itself. This also includes the uncertainty on the imparted dose. Fig. 1 reports the output dose from the X-ray generator as a function of the X-ray exposure time, while Fig. 2 shows the amplitude of the signal scanned from the PSP-IP, for both BAS and ST-V types, as a function of the X-ray exposure time. As it can be seen, the response is very linear, and the uncertainty of measure is probably dominated by the tolerance of measure of the X-ray dose meter.
Fig. 1. 765DC—dose vs. exposure time (65 kV, 7 mA, SDD 30 cm, 0.26 Gy/min).
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Fig. 2. PSP-IP signal vs. dose.
The overall signal response from the imaging plate is subject to more fluctuation due to the inherently greater inaccuracy of the method of measure, but the linearity of the response is confirmed. Also, it is concluded that ST-V type PSP imaging plates are about five times more sensitive (faster) than the BAS type. The result of exposing the precharged imaging plates to light is reported in Figs. 3 –5. The decay curve is not exponential—that is, after a very fast drop, the rate at which further signal is removed tends to level out to a slower fading rate than the exponential.
4. Conclusion The results indicate that the erasing process is not percentually linear with the (light intensity) (light exposure time) product. This is consistent with the prediction of the
Fig. 3. Light erasing at 10,000 lx (BAS IP).
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Fig. 4. Light fading/erasing at 600 lx.
physical model of photo-stimulated phosphors, which involves a multiple electron energy levels decay process, and consequently a high-order kinetics, as reported by various authors [4,5]. The results also demonstrate minor differences in the effectiveness of different types of light (with different spectral distribution) at fading and/or removing the latent image from the imaging plate. A practical (and unwelcome) consequence of this is that, while a significant portion of the signal may quickly fade out if the IP is exposed to light, e.g. to regular office environment light even for a few seconds, the ‘‘complete’’ removal of the residual latent signal (say to less than the least-significant-bit in an 8-bit signal digitalisation) requires exposure to bright light for a significant span of time. The former effect may not easily be manifest to the end user,
Fig. 5. Signal fading at 20 lx (BAS IP).
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because most commercial software for handling images from imaging plates incorporate functions to automatically stretch the available signal to full range. Often this is performed by the device SW driver itself and is transparent to the user. Under this tenet, the visible consequence of signal fading due to undue light exposure is only a deterioration of the signal-to-noise ratio (more ‘‘grainy’’ images). The data confirms the practical need to handle X-ray exposed imaging plates in subdued light environment prior to scanning, and provides guidelines to that effect. The data also provides guidelines for appropriate light intensity and erasing time for complete removal of the residual latent image after a scanning and prior to a new X-ray exposure.
References [1] R. Couture, C. Hildebolt, Quantitative dental radiography with a new photostimulable phosphor system, Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endo. 89 (2000) 498 – 508. [2] K. Takahasho, Fuji Computed Radiography, Technical Review No. 14, Imaging Plate (IP), Fuji Photo Film, Tokyo, 2002. [3] DenOptixR Digital Imaging System—User Manual and Installation Guide, Dentsply Gendex, Des Plaines, IL, USA. [4] C.C. Shaw, J.M. Herron, D. Gur, Signal fading, erasure and re-scan in storage phosphor imaging, Medical Imaging VI: Instrumentation, SPIE, vol. 1651, 1992, pp. 156 – 163. [5] C.J. Hall, R.A. Lewis, B. Parker, J.S. Worgan, Secret life of image plate phosphors, Rev. Sci. Instrum. 53 (1) (1992, January) 697 – 699.
Effect of visible light on photo-stimulated-phosphor imaging plates Roberto Molteni * Gendex Dental X-ray Division, Dentsply Gendex, 901 W. Oakton Street, Des Plaines, IL 60018-1884, USA Received 18 March 2003; received in revised form 18 March 2003; accepted 20 March 2003
Abstract When irradiated with X-ray radiation, photo-stimulated-phosphor (PSP) imaging plates (IP) charge up with a latent radiographic image. Such image is discharged, hence (partially) erased, on exposure to visible light. In this work, the effects of exposure to light are quantitatively investigated and reported, in order to provide proper guidelines for (a) handling radiographically exposed imaging plates prior to scanning, so as to prevent excessive fading of the latent image and loss of information due to ambient light, and (b) removing the remaining latent image prior to utilising the imaging plate for a further X-ray examination. Data are reported for three types of light and two types of photo-stimulable phosphor. D 2003 Published by Elsevier Science B.V. Keywords: Storage phosphor; Imaging plates; Photo-stimulated phosphors; Photo-stimulable-phosphor radiography
1. Introduction and purpose When irradiated with X-ray radiation, photo-stimulated-phosphor (PSP) imaging plates (IP), a.k.a. storage phosphors, charge up with a latent radiographic image. This virtual, latent image is physically constituted by the local trapping of electrons into metastable electronic levels, whose spatial density is directly proportional to the absorbed dose over a very broad range [1]. In the dark, the trapped electrons will ultimately leak from the trapping energetic states and the latent image will naturally fade away over a period of several weeks. However, the image can be quickly discharged, hence (partially) erased, on exposure to visible light. * Tel.: +1-847-640-4922; fax: +1-847-640-4970. E-mail address: [email protected] (R. Molteni). 0531-5131/03 D 2003 Published by Elsevier Science B.V. doi:10.1016/S0531-5131(03)00345-5
1200
R. Molteni / International Congress Series 1256 (2003) 1199–1205
Light from any part of the visible spectrum is variedly effective at erasing the PSP; however, according to data published by the PSP manufacturer [2], the most efficient erasing occurs with light at approximately 600 nm wavelength (red). Imaging plates have been used for many years for medical applications as a part of dedicated scanning systems that handle the imaging plate inside of a light-tight cassette, and that automatically erase the remaining signal or latent image at the conclusion of a scanning. In dental radiology, however, the simpler (and less expensive) systems that have become commercially available during the past few years usually rely on manual handling both for loading the imaging plate into the scanning device, and for erasing any remaining latent image after the scanning is performed. Therefore, the effects of exposure to light need to be quantitatively investigated in order to determine:
The procedures to properly handle X-ray-exposed IP prior to scanning to prevent excessive fading of the latent image, and loss of information, due to ambient light. The procedures to properly remove the remaining latent image prior to utilising the imaging plate for a further X-ray examination.
2. Materials and methods Dental-specific PSP imaging plates (IP) have been used for the investigation:
Fuji BAS in intraoral size #2 Fuji ST-V, which is used for panoramic (15 30 cm, 5 12 in.) and cephalometric (18 24 cm, 8 10 in.) sizes.
Both PSP materials are originally manufactured by Fuji Films, Tokyo, Japan, and finished and branded by Dentsply Gendex, Des Plaines, IL, USA, for use with the DenOptix dental X-ray scanning system. Imaging plates made of the same, or similar, material are used in various other commercial systems, for instance, ScanX marketed by AirTechnique and DigoraR produced by Soredex. In addition to size #2 intraoral imaging plates (which are made out of BAS material), a panoramic-size imaging plate, which is made out of ST-V material, was cut into #2-size plates, for convenience and consistence of the tests. In this way, the panoramic- and cephalometric-specific PSP-IP material could be handled in the same manner as the intraoral-specific PSP-IP material, i.e. be subject to the same X-ray irradiation procedure with the same generator and test object, and scanned and erased in the same way as the (BAS) intraoral IP. The imaging plates were irradiated with a controlled amount of X-ray radiation dose, in a dedicated fixture, which consisted of a mount to ensure repeatable exposure geometry at a source-detector distance (SDD) of 30 cm. The fixture included a very simple test object that consisted of a thin (0.15 mm) straight lamina of lead vertically slanting at a small angle at one side of the exposed field on the imaging plate. This test object is not relevant for the purpose of the investigation reported herewith, except for the convenience of making images immediately recognisable.
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The 765DC dental intraoral X-ray generator (Dentsply Gendex, Des Plaines, IL, USA) was used. The 765DC features 65 kV and 7 mA at constant potential (DC), and was equipped with a 30 cm rectangular collimator (note that the presence and nature of the collimator are irrelevant for the purpose of this test). Due to their design, DC-operated Xray generators can ensure consistent exposure and a very linear time– dose relationship even at short exposure times. The Test-O-Meter type 511 radiation dose meter from Unifors Instruments (Billdal, Sweden) was used for measurements of dose. The imaging plates were scanned with DenOptix (Dentsply Gendex, Des Plaines, IL, USA), SCSI version, and the resulting images were analysed through the VixWin2000 software (Dentsply Gendex, Milan, Italy). This system, if properly configured, can acquire the signal in a strictly linear fashion, and the software itself provides the tools to quantitatively analyse it. Such configuration implies the following menu settings: ‘‘Options\Preferences\8 – 16 bit’’: 16 bit. ‘‘Options\Automatic Image Treatment\DenOptix I/O’’: ‘‘Raw Image’’ on, ‘‘Equalize’’ off. The irradiated images were (partially) erased by exposing to three different types of light sources (with different light spectra), i.e. ‘‘cool white’’ fluorescent light tubes (for general illumination), ‘‘aquarium’’ fluorescent light tubes (purple light), and incandescent filament bulbs. A Gossen Multi-Pro photometer was used for light measurements. The measures were also double-checked with an INS DX-100 Lux-meter. Initially, a test was conducted to determine the reproducibility in the response of the PSP-IP when exposed to consistent X-ray exposure in the measurement setup, and using different pieces of imaging plates. A test was then conducted to verify the linearity of the exposure dose from the 765DC Xray generator as a function of exposure time. Then, the linearity of the response of the PSP-IP as a function of exposure time was measured, for both the BAS type and the ST-V type. The ‘‘response’’ is defined as the ‘‘signal’’ readout of a rectangular region of the exposed and scanned IP, which has been subject to (approximately) uniform X-ray irradiation. This is obtained by complementing to 65535 (64k = 216 1) the mean value of the signal read over the ROI, i.e. the 16-bit digital image ‘‘density’’, due to the fact that the digital image presented is a negative or complementary image (as customary in radiography). Since this test was run in comparable conditions for both PSP-IP types, the result also provides their relative sensitivity. The a.m. tests are just preparatory to the tests that constitute the actual scope of this investigation, i.e. to quantify the effect of visible light to fading and/or erasing the latent signal stored in the PSP-IP. BAS type imaging plates were exposed to a predefined X-ray dose, i.e. 0.87 mGy (obtained with an exposure time of 200 ms in the test fixture). As reported in Section 3, this resulted in a latent signal of about 80% of the full-scale range. The procedure was repeated with ST-V type imaging plates, however, with an exposure time of 40 ms resulting in 0.17 mGy and an image signal comparable to the one above.
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The preexposed IP were then subject to visible light of different spectrum, i.e. from:
‘‘Cool white’’ fluorescent tubes, Radionic Industries type F14T8CW; ‘‘Aquarium’’ fluorescent tubes, Radionic Industries type F14T8AQ. These are purple in colour, with a higher content of short wavelengths; Incandescent filament bulb lamps. Light exposure/erasing was performed at three different light intensities:
10,000 lx, this being a bright light intensity typical of light erasing fixtures, set at very close distance from a bank of fluorescent tubes. Exposure to light was performed in a time range up to 30 s; 600 lx, this being representative of a well-lit office environment. Exposure to light was performed in a time range up to 60 s; 20 lx, this being illustrative of dim, or subdued, light conditions as prescribed by the manufacturer instructions [3] for handling the PSP-IPs after X-ray exposure and prior to the scanning and image readout, e.g. typically achievable in a windowless unlit room where light enters through a door open onto a regularly lit office environment. Exposure to light was performed in a time range up to 300 s. 3. Results At a exposure dose of 0.7 mGy (corresponding to 160 ms) and 12 samples, the average signal was 56.0% of full scale, with a standard deviation of 1.3% of full scale, and 2.3% of the signal itself. This also includes the uncertainty on the imparted dose. Fig. 1 reports the output dose from the X-ray generator as a function of the X-ray exposure time, while Fig. 2 shows the amplitude of the signal scanned from the PSP-IP, for both BAS and ST-V types, as a function of the X-ray exposure time. As it can be seen, the response is very linear, and the uncertainty of measure is probably dominated by the tolerance of measure of the X-ray dose meter.
Fig. 1. 765DC—dose vs. exposure time (65 kV, 7 mA, SDD 30 cm, 0.26 Gy/min).
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Fig. 2. PSP-IP signal vs. dose.
The overall signal response from the imaging plate is subject to more fluctuation due to the inherently greater inaccuracy of the method of measure, but the linearity of the response is confirmed. Also, it is concluded that ST-V type PSP imaging plates are about five times more sensitive (faster) than the BAS type. The result of exposing the precharged imaging plates to light is reported in Figs. 3 –5. The decay curve is not exponential—that is, after a very fast drop, the rate at which further signal is removed tends to level out to a slower fading rate than the exponential.
4. Conclusion The results indicate that the erasing process is not percentually linear with the (light intensity) (light exposure time) product. This is consistent with the prediction of the
Fig. 3. Light erasing at 10,000 lx (BAS IP).
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Fig. 4. Light fading/erasing at 600 lx.
physical model of photo-stimulated phosphors, which involves a multiple electron energy levels decay process, and consequently a high-order kinetics, as reported by various authors [4,5]. The results also demonstrate minor differences in the effectiveness of different types of light (with different spectral distribution) at fading and/or removing the latent image from the imaging plate. A practical (and unwelcome) consequence of this is that, while a significant portion of the signal may quickly fade out if the IP is exposed to light, e.g. to regular office environment light even for a few seconds, the ‘‘complete’’ removal of the residual latent signal (say to less than the least-significant-bit in an 8-bit signal digitalisation) requires exposure to bright light for a significant span of time. The former effect may not easily be manifest to the end user,
Fig. 5. Signal fading at 20 lx (BAS IP).
R. Molteni / International Congress Series 1256 (2003) 1199–1205
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because most commercial software for handling images from imaging plates incorporate functions to automatically stretch the available signal to full range. Often this is performed by the device SW driver itself and is transparent to the user. Under this tenet, the visible consequence of signal fading due to undue light exposure is only a deterioration of the signal-to-noise ratio (more ‘‘grainy’’ images). The data confirms the practical need to handle X-ray exposed imaging plates in subdued light environment prior to scanning, and provides guidelines to that effect. The data also provides guidelines for appropriate light intensity and erasing time for complete removal of the residual latent image after a scanning and prior to a new X-ray exposure.
References [1] R. Couture, C. Hildebolt, Quantitative dental radiography with a new photostimulable phosphor system, Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endo. 89 (2000) 498 – 508. [2] K. Takahasho, Fuji Computed Radiography, Technical Review No. 14, Imaging Plate (IP), Fuji Photo Film, Tokyo, 2002. [3] DenOptixR Digital Imaging System—User Manual and Installation Guide, Dentsply Gendex, Des Plaines, IL, USA. [4] C.C. Shaw, J.M. Herron, D. Gur, Signal fading, erasure and re-scan in storage phosphor imaging, Medical Imaging VI: Instrumentation, SPIE, vol. 1651, 1992, pp. 156 – 163. [5] C.J. Hall, R.A. Lewis, B. Parker, J.S. Worgan, Secret life of image plate phosphors, Rev. Sci. Instrum. 53 (1) (1992, January) 697 – 699.